1. Field of the Invention
The present invention relates to a passive proximity transceiver device of the type used vehicle security systems, and in particular to the casing of such a passive proximity transceiver device.
2. Description of Related Art
Passive proximity transceivers are commonly used in security and convenience systems where it is necessary or desirable for some sort of electronic device to receive some sort of input from a user without requiring the user to perform a specific action. For example, in a vehicle security system that immobilizes the engine's starter, fuel pump, ignition, or other vital circuit, a passive proximity transceiver associated with or connected to the driver's ignition key can be used to send a disarm signal to the security system in response to a query signal from the security system when the driver inserts the ignition key and thereby brings the passive proximity transceiver into proximity with a corresponding transceiver mounted near the ignition switch. The driver is thus able to provide an input to the security system without having to perform any action other than the normal action of placing the key into the ignition in preparation for starting the vehicle. Similarly, the proximity transceiver could be used to send a disarm or other signal to the security system when a key to the door of the vehicle is inserted into a door lock.
In another situation, someone driving up to a gasoline pump in order to purchase gasoline might trigger an infrared or other sensor. The sensor would in turn trigger an electronic device within the gasoline pump to send a query signal to the passive proximity transceiver attached to the driver's key chain. In response, the passive proximity transceiver should send a coded RF signal to the electronic device, causing that device to allow gasoline to be pumped and the cost of the gasoline to be charged to the driver's credit card, the account information for which had been previously programmed into a computer system attached to the electronic device.
The construction of a typical proximity transceiver 1 is illustrated in FIG. 1. The proximity transceiver 1 is designed to be carried by a person and to transmit a coded signal in response to receipt of a corresponding coded signal from transmitter/encoder circuitry 2 mounted, for example, near the door lock or ignition switch of a vehicle, or some other electronic device. Typically, the proximity transceiver 1 includes receiver/decoder circuitry 3 made up of an RF receiver 4, demodulator 5, and comparator 6 for comparing a code extracted from the received signal with a preset code, generated by the decoder circuitry or retrieved from a memory or register, represented by functional block 7. If the extracted code matches the pre-set code, then the transmitter/encoder circuitry 8 is activated to generate a coded signal by using an RF modulator 9 to impress a further code 10 on the RF carrier generated by functional block 11, and a transmitter 12 to transmit the resulting coded signal to a receiver 13 in the electronic device receive/decoder circuitry 2.
The transmitter 12 and other circuitry in the proximity transceiver 1 are powered by a low voltage battery, and the range of the transmitter is on the order of a few inches, so that the transmitted signal is received by the receiver/decoder circuitry 2 only if the proximity transceiver is very close to the receiver 13 of the receiver/decoder circuitry 2. The receiver/decoder circuitry 2 also includes a demodulator 14 for demodulating the coded signal received from the proximity transceiver 1 and comparison circuitry 15, which may be a separate component or part of the controller 16 of a security system or other electronic device 17. Circuitry 15 compares the extracted code with a code generated or retrieved from functional block 18 and, if the codes match, enables the controller 15 to perform a function such as authorizing pumping of gasoline or disarming a security system. The transmitter/encoder circuitry 19 which transmits the initial coded signal that activates the proximity transceiver 1 typically also includes a code generator or source of pre-set codes 20, an oscillator 21 for generating a carrier wave, a modulator 22, and a transmitter 23, and is itself activated by the controller 16 upon receipt by the controller of an input from the security system or other electronic device 17, for example upon turning of a key in a lock.
The transceiver circuitry of the conventional proximity transceiver is enclosed in a case 24, shown in perspective in FIGS. 2A and 2B, and in an end view in FIG. 2C. Case 24 completely encloses the proximity transceiver circuitry, and includes an opening 25 for a key ring 26, so that the proximity transceiver can easily be carried by the user. Conventionally, the casing has the general form of a regular solid such as a parallelpiped, ellipsoid, cylindroid so as take up a minimal amount of space, with a smooth outer surface that makes it easy to grasp, although the shape can include features such as the inclusion of both flat surfaces 27 and curved surfaces 28, the invention being intended to apply to all such conventional casings.
The problem addressed by the present invention is that any metal-containing object in proximity with the transceiver, such as keys held on the same key ring as the transceiver, particularly if the key ring holds keys other than the key being inserted into the ignition or lock with which the transceiver is associated, will interfere with transmission or receipt of the coded signals, causing a drastic reduction in range, or completely preventing a transmission from being received. In order to ensure proper transmission and receipt of signals, as wide a gap as possible must be maintained between the passive proximity transceiver and any metal objects. At present, the only way to maintain such a gap is either for the user to hold the transceiver away from the other keys, or to use a larger case. Having the user manipulate the transceiver defeats the purpose of the transceiver, which is to provide a coded input that does not require user intervention, while making the case larger makes it inconvenient to carry and increases the cost of the device.